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RESEARCH PAPER OPEN ACCESS

Interactive effects of arbuscular mycorrhiza fungi Glomus

intraradices and Trichoderma harzianum against Fusarium wilt

of tomato

Lilian Wanjiru Mbuthia1, Leonard Muriithi Kiirika2*, Gloria Afolayan3, Von Alten

Henning1

1Institute of Horticultural Production Systems, Section Phytomedicine, Leibniz University

Hannover, Hannover, 30419, Germany

2Department of Horticulture and Food Security, Jomo Kenyatta University of Agriculture and

Technology. P.O. Box 62000 00200 Nairobi, Kenya

3National Center for Genetic Resources and Biotechnology (NACGRAB), Moor Plantation, PMB

5382, Oyo State, Ibadan, Nigeria

Key words: Arbuscular mycorrhiza fungi, Biological control, Trichoderma harzianum, Fusarium oxysporum f.

sp. lycopersici, Synergistic.

http://dx.doi.org/10.12692/ijb/15.1.251-268 Article published on July 18, 2019

Abstract

Biological control agents (BCA) are important as some establish symbiosis with plants hence controlling plant diseases,

improving plant nutrients uptake and water absorption. Use of BCA in soil borne disease management is not fully harnessed

and is also faced with inconsistencies in developing their formulations. We therefore investigated the use of arbuscular

mycorrhiza fungus (AMF), Glomus intraradices, and Trichoderma harzianum (T-22) against soilborne pathogen Fusarium

oxysporum f. sp. Lycopersici (Fol) in tomato. G. intraradices isolate 510 held on expanded clay as carrier material was

incorporated into the substrate during germination of tomato seeds and at the transplanting stage. T-22 inoculum was also

initiated from potato dextrose agar and inoculated at each transplanting stage, while Fol was applied through drenching. To

test the possible synergistic effects, AMF and T-22 were applied in combination under varying niches. Results showed that

application of AMF and T-22 together had significant reduction (30.5% p

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Introduction

Fusarium oxysporum is an ubiquitous soilborne

pathogen that includes various pathogenic strains

which lead to economic losses in tomatoes

(Solananum lycopersicum) (Gordon and Martyn,

1997; Fravel et al., 2003; Di Pietro et al., 2003).

Chemicals applied to control Fol have been

unsuccessful as they barely target their ecological

niches but also pose dangers to the environment

(Lucas, 2006). Abuscular mycorrhiza fungi is shown

to present bio-protection effects against fungal

phytopathogens including Phytophthora, Fusarium,

Pythium and Rhizoctonia (Pozo et al., 1996; Cordier

et al., 1998; Akköprü and Demir, 2005). Its

ubiquitous nature is a fundamental component for

development, exploitation and adaptation to

interactions with the other rhizosphere microfloral of

which knowledge of mode of interactions can be

harnessed for development of an effective biological

control strategy (Linderman, 1994; Fillion et al.,

1999; Vazquez et al., 2000). Trichoderma is a

soilborne fungus commonly used as an antagonist

against fungal pathogens and as a bio-pesticide,

which is exploited for its biological control potential

(Harman, 2000; Harman et al., 2004). Moreover,

control may be achieved indirectly by inducing host

plant resistance where not only microbial organisms

have been used but also compounds that mimic their

cellular components to induce resistance (Kiirika et

al., 2014).

The efficacy of these biological control agents and

consistence performance in controlling soil borne

diseases are the hurdles that must be overcome if

their application is to be widely used even for

commercial purposes. Attempts to combine

alternative treatments to improve performance have

been made but their application is faced with similar

constraints as the level of protection is influenced by

factors such as the pathogen strain, environment, soil

type and nutrients (Guetsky et al., 2001; Larkin and

Fravel, 2002; Spadaro and Gullino, 2005). Optimal

plant nutrition is essential for a successful disease

management where manipulation of mineral

nutrients such as nitrogen, phosphorous, sulphur and

potassium have been shown to contribute to control

of various plant diseases. Phosphorus (taken up by

plants as H2PO4– and HPO42-) is shown to have

multiple roles not only as an essential plant nutrient

but also mitigates disease development in plants

(Sweeney et al., 2000). High levels of phosphorous in

the soil however shown to reduce host’s ability to

develop AMF symbiosis (Balzergue et al., 2013). In

the current study, we investigated the interactions of

AMF and T-22 under varying phosphorous regimes

aimed at controlling Fol. Tests to determine control

effects of AMF and T-22 against Fol, tests were

carried out under different substrates containing

different P regimes. We hypothesized that application

of AMF and T-22 inhibits Fol infection and the effects

will be synergistic for combined applications.

Materials and methods

Plant materials and culture conditions

Tomato seeds of cultivar Cuor Di Bue were raised in

trays containing quartz sand and transplanted after

10 days into 9 cm diameter plastic pots (volume 230

ml). Six weeks later, seedlings were transplanted into

bigger pots of 17cm diameter (volume 2 l) and kept

under greenhouse conditions (20 oC day/night, 12 h

light per day/30 K Lux and 70% RH) and watered

regularly to the soil field capacity.

Microbial culture and inoculation

AMF and T-22 isolates were taken from the collection

of the Institute of Horticultural Production Systems,

section Phytomedicine. The inoculum of G.

intraradices isolate 510 on expanded clay as carrier

material (Dehne and Backhaus, 1986) was

incorporated into the substrate during germination

and at each transplanting stage at 5 % (v/v). T-22

inoculum was initiated from potato dextrose agar,

cultured into liquid conical flasks with sterile media

consisting of perlite pre-soaked with 2 % malt extract

solution (Kraftnahrung rein, from Villa Natura), at

2 % (v/v) into the culture substrate at each

transplanting stage. Fol inoculation was done through

drenching by pouring direct into the pot at 30 ml

spore suspension (1.5 x 108 spore/ml) at the second

transplanting stage.

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Substrate and fertilization

Pure quartz sand with and without 30 % (v/v) fine

ground white peat from Lithuania (calcium carbonate

at 500 g/100 l added to increase pH) were used as

substrates. Fertilization was carried out in two

nutrient regimes; Hewitt (Hewitt, 1966) low

phosphorous (LP), by 12.85 g/l NAH2PO7.H2O) and

Hewitt high phosphorous (HP) (128.5 g/l

NAH2PO7.H2O).

Experimental setup

Interactions between AMF and T-22 and their effect

when applied together on Fol were tested using

tomato cultivar Cuor Di Bue. Experiments were

conducted under greenhouse conditions, 16 h light

per day with additional light using Phillips lamps SRG

102/400 (195 µmol sec-1 m-2) during winter. Disease

progress was monitored on tomato plants growing

with temperature maintained at 22 °C (day) and 18 °C

(night) (Fig. 1). The experiment consisted of

treatments arranged in factorial design with Fol

infected or not infected (control) and subjected to the

two P-levels and substrates: (i) without T-22 or AMF

(control), (ii) with AMF alone, (iii) with both T-22

and AMF, (iv) with T-22 alone. Two conditions were

compared, during summer and winter periods which

enabled identification of differences due to changes in

different environmental conditions of the two

seasons. Furthermore, we investigated the influence

of P on Fol disease symptom expression. To observe

whether plants would start showing yellowing

symptoms of Fusarium wilt when supplied with more

P, plants with LP for AMF and control but only in the

substrate sand were included in the setup with the

aim of increasing the level of P in this extra set of

plants from LP to HP after the initial wilt symptoms

were observed.

Assessments and data collection

To assess the effects of different treatments,

parameters including plant growth, disease

development, severity and incidence were considered.

AMF and T-22 establishment and intensity were

observed at different plant growth stages to monitor

their development and possible interacting effects.

Analysis of P content in the plant tissue was carried

based on the phosphorous acid colorimetric

determination method with ammonium molybdate

vanadate by Gericke and Kumries (1952) to decipher

differences in its content based on the P nutrition

level. In order to monitor disease development and

severity, visual symptoms were assessed. Symptoms

of Fol infection were observed including wilting and

yellowing and each scored separately using a scale of

0-4 according to the following classes: 0=no

symptoms, 1=< 25 % leaves with symptoms, 2=26-

50 % leaves with symptoms, 3=51-75% leaves with

symptoms and 4= 76-100 % leaves with symptoms.

Confirmation of disease incidence and development

in the plants were carried out according to

Grunewaldt-Stöcker (1994): Fol was re-isolated from

the stem cross sections at different heights (stem

base, 25 cm and 50 cm). The outer surface of the stem

was sterilised with 70 % ethanol and thin cross

sections cultured on Komada Agar (Komada, 1975).

The growth of the mycelia from the vascular bundles

was then assessed with 1/3, 2/3, 3/3 reflecting the

number of infected bundles. Re-isolation of Fol from

the substrate was done using Komada agar following

the basic dilution-plating technique: 15g of the

substrate was diluted in 150 ml sterile water, stirred

for 10min (750 rpm), a defined volume of a 10-3

dilution was then plated out on Komada agar in 3

replicates, and then incubated for 5-6 days at 23 °C

after which colonies formed per plate were counted.

This was only done for the AMF/T-22 sand HP

treatment for the first experiment as majority of

plants in the treatment showed no symptom

development and was therefore necessary to confirm

the success of inoculation by determining the

pathogen presence in the rhizosphere.

Trichoderma infection and intensity estimation

T-22 was estimated from the rhizosphere dilution-

plating at the second transplanting and harvest

stages. Briefly, quantitative isolation of T-22 was done

using selective culture medium (Steinmetz, 1994 -

unpublished): 15g of the substrate diluted in 150 ml

sterile water, stirred for 10 minutes (750 rpm) and a

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defined volume of a 10-3 dilution was then plated out

on TSA agar (Lewis and Papavizas, 1984) in

triplicates. The culture was then incubated for 5-6

days at 23 °C and colonies formed per plate counted

as colony forming units (CFU)/g substrate using the

following formula:

AMF colonisation assay

AMF colonisation was quantified also at the second

transplanting and at harvest stages. Samples for the

assay were collected from washed root systems by

cutting a transverse band of about 1cm width at

around 5cm below the root crown. Staining was done

following Vierheilig et al. (1998): roots were soaked in

3 % KOH overnight, washed several times with tap

water, soaked overnight in ink-vinegar-solution (1 %

ink and 5 % household vinegar in a dest.). Estimation

of AMF colonisation was then done under a light

microscope (Axiolab, Zeiss) using a scale from

Backhaus (1984) as follows: 0 = no infection, 1-up to

30 % of the root section colonized, 2-up to 60 % of the

root section colonized and 3-colonisation of the whole

root.

Statistical analysis

The statistical analysis was done using the R software,

version 2.9.0. To determine the interactions with

regard to the effect of phosphorous, substrate, and

biological control agents, a fit of an ordered logistic

regression model was used.

The model worked at estimating the probability of an

individual to get a score of 0, < 1, < 2, or < 3 for the

AMF establishment. Parameter estimates were the log

difference of odds, comparing the factors substrate,

phosphorous and the treatments. The confidence

intervals (CI) at 95 % were then calculated for the

parameters and a significant difference was shown if

the value 1 was not included in the interval. A Chi-

square test of independence with multiplicity

adjustment by Bonferroni was then done at a p-value

≤ 0.05. The model was also used to plot a model

demonstrating the differences in symptom

development in terms of wilting and yellowing as

influenced by phosphorous level at the probabilities

of getting the different scores of 1, 2, 3 or 4 and is

shown in the results segment at the probability of

getting a score of 3. For the analysis of disease

severity and Trichoderma establishment, the

treatment effects were tested using analysis of

variance (ANOVA). The significant differences were

then tested by Tukey multiple range tests (p-value

≤0.05). For disease severity, the area under disease

progress curve (AUDPC) was calculated using the

formula below and data subjected to ANOVA.

1

1

11 2/n

i

iiii ttxxAUDPC

Where, xi and xi-1 are the wilt index, and ti and ti-1 are

consecutive evaluation dates (with ti-ti-1 is equal to 1

day).

Results

Establishment of AMF and T-22 under different

substrates and P-Levels

Analysis of interacting effects of AMF and T-22 as

well as the influence of different substrates (sand and

sand-peat mixture) and P-levels (low P and high P)

revealed that P level had a significant influence on

mycorrhiza colonisation with the logistic model

showing differences regardless of substrate, with or

without T-22.

Fig. 1. Tomato plants under greenhouse conditions

showing Fol wilting symptoms. Plants inoculated with

Fol (right) healthy plants without Fol inoculation

(left).

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This effect of P on mycorrhiza colonization was

evident both at the initial establishment (after 6

weeks) with a confidence interval (CI) of 3.38-16.62

and at harvest (after 13 weeks) CI of 10.02 - 23.28. A

Chi-square test of independence revealed high

significant differences in mycorrhiza colonization for

treatments under LP compared to treatments under

HP (Fig. 2A and B).

Fig. 2. AMF colonisation is influenced by different P levels (low and high levels). The graph shows data of mean

frequency values ± SD; n= 8; after inoculation with AMF alone and in combination with Trichoderma (AMF/T-

22) in the different substrates sand (SAND) and sand peat (S.PEAT) after (A) six weeks and (B) 13 weeks.

Significant differences between LP and HP in each treatment are indicated by (*) according to Chi-square test of

independence (p-value ≤0.05).

The substrate type also had significant effects on AMF

colonization with plants grown in sand showing

higher colonisation compared to sand peat at 6 weeks

(Fig. 3A and B) CI (1.18 -6.41) and after 13 weeks (Fig.

3C and D) CI (1.83-4.43). At 6 weeks, these

differences were clearly seen in the AMF/T-22 in low

P having significantly higher mycorrhiza colonisation

under sand substrate compared to sand peat (Fig.

3A). At 13 weeks, high influence was under the AMF

treatment in HP having significantly higher

colonisation in sand compared to sand peat (Fig. 3D).

AMF colonisation appeared not to be affected by the

addition of T-22 which is evidenced by almost similar

AMF-colonisation pattern levels at both the combined

treatment AMF/T-22 and individual AMF treatment

(Fig. 4A-D). The only significant difference was

detected at 6 weeks (Fig. 4B) whereby the combined

application of AMF/T-22 shows higher colonisation

in the substrate sand at LP-level.

Substrate was observed to have significant effect (p <

0.05) on T-22 establishment (Fig. 5). Treatments

under sand-peat had significantly higher levels of T-

22 compared to treatments under sand regardless of

P levels both at 6 weeks (Fig. 5A and B) and at harvest

(Fig. 5C and D). P level and AMF did not show

significant effect on T-22 establishment.

Effects of AMF and T-22 on Fusarium wilt

development under different substrates and P-levels

To determine the biological control effects of AMF

and T-22 on Fol when either applied alone or in

combination (AMF/T-22), a mixed model ANOVA

analysis of the AUDPCs under all substrate and P

levels was done. Results by ANOVA revealed

significant effect of substrate with treatments under

sand-peat which had significantly higher disease

severity compared to treatments under sand

regardless of P-level or biological control treatment

(Fig. 6A and B).

c)

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Fig. 3. AMF colonisation is influenced by substrate sand peat and sand within the same P levels. The graphs

shows data of mean frequency values ± SD; n= 5 and 8 at (A, B) 6 weeks and (C, D) 13 weeks respectively; after

inoculation with AMF alone and in combination with T-22 (AMF/T-22) at equal levels of phosphorous.

Significant difference between the substrates Sand peat (S.Peat) and Sand in each treatment are indicated by (*)

according to Chi-square test of independence (p-value ≤0.05).

Results are also presented as disease progress curves

for the treatments with T-22 and AMF at LP or HP

both during summer and winter periods (Fig. 8)

where the developmental trend was similar for all the

treatments.

A Tukey’s multiple range tests was then done to

compare treatments within the same substrate and

under the same P-levels (Fig. 8A-H) While no

significant differences were detected in the substrate

sand peat at either HP or LP, the disease progress

curves show the combined treatment of AMF/T-22

having the lowest severity level compared to the

control (Supplementary fig. 1A and B) The most

pronounced and significant effect was observed in the

substrate sand with HP where the combined

treatment AMF/T-22 showed a significant lower

disease severity compared to all the other treatments

(Fig. 1C) Re-isolation of the pathogen from the

substrate of this particular treatment confirmed

pathogen presence in the treatment (data not shown).

In the sand substrate at LP, no significant differences

were detected between the treatments with no clear

trend in the disease progress curves, but in the AMF

treatment, the trend was lower compared to others

(Fig.8D). The re-isolation of the pathogen incidence

from the stems from all the treatments inoculated

with Fol was consistent with the visual wilt

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development scores (data not shown) with the plants

showing symptoms visually also showing infection in

the selective agar media, while those that did not have

any symptoms visually not showing any infection.

The second experiment was done in winter and

allowed us to observe the differences due to different

environmental conditions as compared to those of

summer in the first experiment. The greenhouse

temperature during summer period ranged from 28-

32 °C while in winter, temperature was maintained

between 22-25 °C. Establishment of AMF and T-22

was again determined at initial stages and at harvest

but this was only done to confirm their establishment.

For T-22, this was done in both treatments containing

T-22 and in the control to determine whether there

was any indigenous Trichoderma spp. in the

substrates used. The presence of T-22 in the

rhizosphere of treatments inoculated with T-22 was

confirmed, but no indigenous T-22 was detected in

the controls. The colonisation of AMF on the roots

treated with AMF was also confirmed.

Fig. 4. AMF colonisation is influenced by addition of T-22. The graphs shows data of mean frequency values ±

SD; n= 5 and 8 at (A, B) 6 weeks and (C, D) 13 weeks respectively; after inoculation with arbuscular mycorrhiza

fungi (AMF) alone and in combination with Trichoderma (AMF/T-22) at the same phosphorous levels within the

same substrate, Sand peat and Sand. Significant difference between AMF and AMF/T-22 at the same

Phosphorous level (LP or HP) is indicated by (*) according to Chi-square test of independence (p-value ≤ 0.05).

In general, the disease incidence and severity levels in

winter were lower than in summer and so was the

progress of disease development which took four

weeks in winter as compared to only two weeks in

summer. Due to the fact that the incidence and

severity levels were very low in winter (Fig. 8E-H)

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with only half of the plants within each treatment

showing symptoms, no statistical analysis was

possible and therefore only the trends of the disease

progress curves are compared. The disease progress

curves indicate that the biological control agents AMF

and T-22 alone or combined also reduced the severity

levels of the disease in winter as their progress curves

are lower in comparison to the controls. The other

trends on the effect of P and substrate on disease

development was the same as in the first experiment.

Fig. 5. Colony forming units (CFU) of T-22 as influenced by substrates, P and AMF. Included are the ± standard

deviation; n= 5 and 8 at (A, B) 6 weeks and (C, D) 13 weeks respectively, after inoculation with T-22 alone or in

combination with AMF (AMF/T-22) under the same substrate Sand or Sand peat and P levels ( LP or HP) at 6

weeks and at 13 weeks.

Effect of P on Fusarium wilt development

From our investigations, we observed an unexpected

influence of P on fusarium wilt symptom

development. It was interesting to note that plants

grown under low P did not develop the typical

expected symptoms associated with Fusarium wilt. A

prominent and distinctive symptom development was

observed between plants under HP and LP. Plants

under HP developed typical Fol symptoms of first

yellowing then gradually wilting while those under

low P directly wilted without yellowing.

This difference was clear across the whole data set

regardless of the treatment or substrate. To

demonstrate this, the scores of both the wilting and

yellowing symptoms were fit into a logistic regression

model showing the probability to get a score of 1, 2, 3

or 4 within the period of disease development.

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Fig. 6. Disease severity is influenced by substrate within the same P levels. The figure shows disease progress

curves within 18 days for treatments with T-22, at (A) low phosphorous level ; (B) high phosphorous level.

The model was then able to show the disease progress

curves of both wilting and yellowing and how this was

influenced by the P level across the whole data set.

This is best shown and illustrated here at the

probability of getting a score of 3 (Fig. 9) and was

clearly observed on plants (Fig. 10).

Fig. 7. The P concentration in plant tissue in treatments with high HP and LP level. Included are the ± standard

deviation; n= 10 in treatments with AMF and without any biological control (control) under the same substrate

sand or sand peat (S.Peat). Significant differences in HP and LP within the same treatment are indicated by (*)

according to Tukey multiple range test (p-value ≤ 0.05).

Plant tissue analysis of control and AMF treatments

was done to determine actual differences in P content

for the treatments with HP and LP. The ANOVA

showed interaction effects at P and substrate with no

interactions at the treatment levels. This is shown in

(Fig. 7) where HP has significant higher

concentration levels than LP. The substrate

interaction effect was brought out by a significant

difference between the treatments in sand having

higher concentrations than those in the sand peat at

HP.

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Fig. 8. The effect of AMF and T-22 inoculations on disease severity in summer and winter periods. The figure

shows disease progress curves for treatments with AMF, T-22, their combination AMF/T-22 and without AMF or

T-22 (control) as they occurred in summer (A, B, C, D) and winter (E, F, G, H) in treatments within the same

substrate and P-levels.

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To further ascertain the effect of P on Fusarium wilt

symptom development and expression, a sub-

experiment was set-up. A sub-set of treatments (AMF

and control under substrate sand) were first grown

under LP levels, inoculated with Fol and upon

symptom development, the P level was then increased

to the HP level. The idea was to increase the level of P

to that of HP after the initial wilt symptoms were

detected and observe whether these plants would

start showing the yellowing symptoms with the P

increase. The disease development period was not

long enough i.e. 18 days from initial symptom

development to maximum severity level to clearly

show the effect of increasing the P level. Within that

period, fertilization with the increased level of P had

been done only two times. Nevertheless, the

observations made towards the end of the experiment

showed that, after increasing the level of P, plants

began to wilt and started showing yellowing

symptoms too.

Discussion

Synergistic effects of AMF and T-22 by dual

inoculation against Fusarium wilt of tomato, with the

aim to contribute to development of effective

biological control strategies against soil-borne

pathogens. Preliminary investigations on the

influence of the two biological control agents on each

other based on establishment were conducted and no

antagonistic effects were observed. Establishment

levels of T-22 and AMF were similar in the dual

inoculation in comparison to their individual

inoculation implying that both organisms have a non-

competitive growth.

Fig. 7. A logistic regression model showing the distinct disease symptom expression (wilting and yellowing) as

influenced by P levels. Different treatments starting from top to bottom which include; (a) T-22, (b) Control, (c)

AMF/T-22 and (d) AMF. The columns indicates the P level and substrate starting from the left with; Sand Peat

HP, Sand Peat LP, Sand HP and Sand LP. The straight line and the dotted lines represent the yellowing and

wilting curves, respectively.

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The trend of wilt symptom development shows that

the combined treatments had lower severity levels

than the individual treatments as well as the control.

This may be attributed to synergistic or additive

effects between AMF and T-22. Synergy between

mycorrhiza and Trichoderma spp. on enhancement of

growth and biological control effects were reported

earlier (Camprubi et al., 1995; Datnoff et al., 1995;

Fillion et al., 1999).

The most significant and conspicuous observation

was observed with sand the sand substrate with HP

whereby the combined treatment of AMF/T-22

showed significantly minimal disease infection

compared to the single inoculated treatments or the

control. Re-isolation of the pathogen from the

substrate of this particular treatment confirmed its

presence, while the analysis of the pathogen incidence

from the stems gained no growth on the selective agar

media. This indicates mechanisms other than

competition and mycoparasitism. Thus, a possibility

of synergistic induction of resistance in the dual

application could have played a key role which

remains to be investigated. Cross-talk between AMF

and Trichoderma during interactions with the host

plant affects disease resistance through their

influence on phytohormone synthensis or transport.

Martinez-Medina et al. (2011) reported that the co-

inoculation of AMF and Trichoderma in Cucumis

melo increased the hormonal profile of SA and JA

that plays a key role during plant resistance responses

as compared when applied singly. Thus, from our

observations, the AMF colonisation took place

congruent to pathogen attack which combined with

the presence of T-22 resulted in the minimal disease

infection observed in the AMF/T-22 sand HP

treatment. However, the results give no evidence that

mycorrhiza was essentially responsible for that effect,

as there was no difference in AMF colonisation

compared to sand peat treatment. Also, the presence

of T-22 in lower density in sand than in the sand peat

also gives no real explanation for the increased

disease suppression. Consequently, it would only

suffice to conclude that other components and/or

factors of that rhizosphere condition may have

enhanced or complimented the defence mechanisms,

the speculations that remain to be investigated.

The enhancement of the resistance levels caused by

both AMF and T-22 towards attainment of high

resistance has been investigated using Trichoderma,

where transformants with over-expression of

chitinase genes were shown to confer higher defence

(Howell, 2003). Improved biological control of Fol by

AMF has also been achieved by the combination of

AMF with hormonal elicitors like salicylic acid and

jasmonic acid (El-Khallal, 2007). On the other hand,

research on the effect of soluble substances released

by extra-radical mycelium of AMF (Fillion et al.,

1999) showed evidence that these substances have

some stimulatory effect on the growth of

Trichoderma. So, the question would be whether

there is also a possibility that AMF may enhance the

production of lytic enzymes in Trichoderma which

can be studied via molecular techniques.

Furthermore, the speculative influence of T-22 and

AMF on each other require also further investigations

either by: a) comparing the dual application of

AMF/T-22 to that of a Trichoderma transformant e.g.

without chitinase with the assumption that it`s

production would be the prominent mechanism of

action leading to the highly induced mechanism in

the dual application or, b) Comparing the resistance

conferred by application of AMF/T-22 to that of

AMF/JA-SA or to that of a transgenic resistant variety

by monitoring the gene expression.

Our results showed that application of P influences

Fol disease development as manifested by reduced

symptom expression. This could not only be

attributed to the function of P as an essential mineral

element required by plants but also its indirect role in

mitigating disease development. Thus, healthy and

vigorous root system will better compensate for

infections especially by roots pathogens. Plants under

low P application exhibited higher disease severity

and wilted directly without the typical Fol symptom

development process involving yellowing of leaves as

observed under high P. P treatment was shown to

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influence the population of Fusarium spp. by affecting

the structural population density, virulence and

disease incidence levels (Woltz and Jones, 1973;

Yergeau et al., 2006). Yergeau et al. (2006) reported

the effects of varying P regimes on Fusarium crown

and root rot of Asparagus officinalis, where disease

incidence was significantly low for plants under low P.

Direct wilting of plant under low P regime further

shows the essential role of P during the plant’s

metabolic processes. As an essential mineral element

that functions as plant structural compound, P is

shown also to act as a catalyst in various biochemical

reactions in plants (Marschner, 1995; Raghothama,

1999; Raghothama, 2000; Walters and Bingham,

2007). Under optimal supply, P is taken up in the

form of orthophosphate (Pi) and translocated into the

cytoplasm (Jain et al., 2005). However, under low P

supply in the soil, the intracellular Pi can be reduced

significantly in the cell affecting the supply of key

energy carrier molecules such as ATP/ADP that are

essential for plant cellular biochemical processes such

as photosynthesis (Marschner, 1995; Rao, 1997;

Raghothama, 1999; Jain et al., 2005). Low Pi

concentration in the cytoplasm affects photosynthetic

process by suppressing the expression of light-

regulated psbO and psbP genes that encode oxygen-

evolving proteins of PSII complex (Jain et al., 2005).

Thus, due to the suppressive effects of P deficiency in

the cells, physiological adaptive changes that

culminate to hampered photosynthesis occurred

leading to direct wilting without yellowing.

Fol infection was shown to limit the photosynthetic

rates of tomato leaves by decreasing the light-

saturated rate of CO2 assimilation accompanied by

decreased maximum carboxylation velocity and in

turn reducing the maximum quantum efficiency of PS

II (Nogues et al., 2002).

Results on plant tissue analysis showed that the

concentration of P was significantly reduced in plants

grown under low P as compared to high P infected

with Fol. Pi is critical for energy metabolism in the

host plant which implies that processes such as

photosynthetic activity were interfered with.

Yellowing of leaves is usually associated with

chlorophyll breakdown, which is an elaborate and

highly energy consuming process (Matile et al., 1996).

Since plants with low P are expected to have a lower

metabolic energy due to the significantly reduced

photosynthesis, this process would cause a

physiological blocking of the chlorophyll breakdown

in a bid to save energy, resulting in expression of

Fusarium symptoms only by wilting.

The role of P was further shown by replenishing the

LP plants with additional P in order to attain the HP

status but after the onset of initial symptoms.

Interestingly, a similar process of direct wilting as for

the plants under HP was observed where plants first

showed the yellowing symptoms (data not shown).

Thus, P application delayed wilting symptoms as

observed also with AMF treatment. Previous studies

showed that the effects of low Pi on photosynthesis

can be reversed by increasing the levels of P to the

optimum (Rao, 1997; Jain et al., 2005). Moreover,

Jain et al., (2005) observed that increased P content

lead to the up-regulation of the genes associated with

photosynthesis that had initially been down regulated

due to P deficiency. Precise mechanisms underlying

the effects of P on Fusarium symptom expression due

to P deficiency is not clearly understood. Molecular

and proteomic tools could possibly be used in future

studies to understand the cellular metabolic activities

ensued upon Fusarium infection, where potential

genes or protein components that most likely support

the possible physiological adaptive mechanisms could

be characterized.

Conclusion

Results of our investigations shows that AMF and T-

22 can co-exist and interact with each other in the

rhizosphere without having any detrimental or

antagonistic effects on their establishment.

Furthermore, we report also the synergistic effects of

AMF and T-22 against the soil-borne pathogen Fol

which contribute to development of soil borne disease

management strategies. Application of AMF in

combination with other biological control organisms

can further be studied to reduce their variability in

264 Mbuthia et al.

Int. J. Biosci. 2019

efficacy and determine the appropriate application

rates.

This has also been shown in several other studies with

AMF, whose effects are shown to be enhanced either

by combining with biological control organisms or

elicitors like salicylic acid and jasmonic acid.

However, interacting factors that affect and

manipulate their efficacy remain to be elucidated in

order to exploit the full potential of the biological

control organisms with the keen interest of

conserving the environment by reducing pesticide use

in cropping systems. Our study shows that substrates,

P nutrition as well as environmental conditions

influences the establishment, growth and

development of the biological control agents used as

well as that of the pathogen.

These findings can be used in predicting how these

biological control agents would develop under certain

conditions that could be used to understand the

functioning of different biological systems, hence

facilitating development of highly specific and reliable

biological control strategies.

Acknowledgements

This research was supported by the Institute of

Horticultural Production Systems, Section

Phytomedicine, Leibniz University Hannover,

Germany.

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